Oxygen barrier resins such as saponified ethylene-vinyl acetate copolymer (hereinafter referred to as “EVOH”) have low oxygen permeability and yet can be adversely affected by high humidity and are liable to undergo an increase of oxygen permeability under a highly humid atmosphere. Generally, packaging structures are adapted to retain their oxygen barrier property by including a protective layer, such as of a polyolefin or some other barrier polymer having a low water absorption or low moisture absorption on a layer of an oxygen barrier resin of high humidity dependency thereby precluding direct exposure of the oxygen barrier layer to water or moisture.
The dependence of the performance of a packaging structure on humidity is an important property because packages are used to store mammal foodstuffs, generally for humans or pets. Often packages are subjected to a “retort process” in which the molded package/container is subjected to excessive moisture as steam and at elevated temperatures typically around 250° F. (around 130° C.) and the ingress of moisture through the outer protective layers exponentially increases with temperature. This effect leads to lowering of the efficiency of EVOH as the barrier, an effect called “retort shock.” Although this lowering is almost a reversible process and the barrier efficiency generally returns almost to its pre shock value, in order to meet the expectations of the shelf life for the food contained in the retorted container the thickness of the EVOH barrier layer must be increased, or lower ethylene mole percent EVOH must be used. These are both expensive alternatives and the most economical way to ensure shelf life is still to protect the EVOH layer from the moisture/steam.
The present inventors have made the unexpected discovery that addition of certain fillers in an effective level to a protective layer results in improved protection of the oxygen barrier component of a packaging structure from moisture.